1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a data session setup system for a wireless network.
2. Description of Related Art
Wireless communications systems include conventional cellular telephone systems which comprise a number of cell sites or base stations, geographically distributed to support transmission and receipt of voice-based communication signals to and from cellular telephones, often referred to as mobile units or wireless units which may actually be stationary or fixed. Each cell site handles voice communications over a particular region called a cell, and the overall coverage area for the cellular telephone system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives forward-link signals from and transmits reverse-link signals to (at least) one cell site or base station. Each active wireless unit is assigned a forward link channel on which it receives its forward link signals and a reverse link channel on which it transmits its reverse link signals. There are many different schemes for defining wireless channels for a cellular telephone system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, the baseband data sent between the base station and the wireless unit is multiplied by a spreading sequence, such as a pseudo-noise (PN) code which is a binary sequence that appears random but can be reproduced by the intended receiving station or unit. As such, different wireless channels are distinguished by different spreading sequences that are used to encode different voice-based streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular voice-based stream from a received signal using the appropriate spreading sequence to decode the received signal.
Due to the delay-intolerant nature of voice communication, wireless units in conventional cellular systems transmit and receive over circuit switched links between a wireless unit and a base station as part of a circuit switched path between the wireless unit and another device, such as a landline telephone. A circuit switched path is a dedicated communications path established between the wireless unit and another device and has a relatively fixed amount of bandwidth. Since the paths are dedicated to the users with a relatively continuous throughput, there is no noticeable delay in transmission. Thus, voice-based systems rely on dedicated circuit switched links to provide continuous streams with fairly uniform bit rates to prevent unacceptable quality of service.
In the context communications between the wireless unit and the base station, a circuit switched link is a dedicated communications path established between the wireless unit and the base station. A circuit switched link also has a relatively fixed amount of bandwidth and can be a portion of a circuit switched path between the wireless unit and another device. While the wireless unit is using the circuit switched link, no other wireless unit can use that link. The circuit switched link is maintained for the duration of the call, and although the wireless unit can hand-off to other base stations, a circuit switched link (a dedicated wireless channel) is maintained between the wireless unit and the base station to service the call. Generally, each active wireless unit requires the assignment of a circuit switched link on the forward link and a circuit switched link on the reverse link.
Traditional data applications are typically bursty and, unlike voice communications, relatively delay tolerant. A block of data is sent followed by inactivity, and so long as the data is not corrupted, a short delay, for example on the order of seconds, may be acceptable. As such, circuit switching or using circuit switched links to transmit data is an inefficient use of network resources. Data transmission relies on packet switching to more efficiently use network resources. Packet switching networks, such as frame relay, asynchronous transfer mode (ATM), and Internet Protocol (IP), share network resources rather than dedicating network resources to a particular user. For example, the packet switching network can use a single packet switched path to forward packets of data from different users over the packet switching network. A data packet (“packet”) is a finite set of data having a predefined protocol and organization. The packet is forwarded over the packet switching network based on a unique address contained in the packet header to deliver the packet message in the body of the data packet. As such, data packets with different destinations can share the same packet switched path.
Wireless communication systems are evolving from conventional voice systems to provide a wide range of wireless applications. For example, some cellular communication systems, such as those conforming to the IS-95B standard or wideband CDMA standards, such as the CDMA2000 and the WCDMA standards, or TDMA Packet Data standards currently being developed, will support wireless units that transmit and receive signals other than just voice-based signals. To provide efficient wireless data communications with packet data networks, wireless communications systems take advantage of the inherently bursty and delay-tolerant nature of the data traffic to more efficiently use wireless resources, such as wireless channels. Accordingly, next generation cellular systems will use packet switched links between wireless units and base stations to establish packet switched connections with a packet data network (PDN), such as the Internet, using packet data services, such as World Wide Web (the Web).
FIG. 1 shows a general block diagram of a wireless communication system 100 with access to a public switched telephone network 102 (PSTN) and a packet data network 104. The wireless system comprises a set of interconnected mobile switching centers (MSCs) 106, each supporting a number of cell sites 108. A wireless unit 110 can establish a voice call using a circuit switched link between the wireless unit and the base station as part of the circuit switched path with another device, such as a wireless unit 110 or a landline terminal in the PSTN 102. For example, a circuit switched link can be on a wireless channel, such as a forward traffic channel, between the wireless unit 110 and the base station 108. If the wireless unit is making a voice call, a circuit switched link is established for the forward link and the reverse link between the wireless unit 110 and the and the base station 108. In current cellular CDMA systems, a forward fundamental code channel is established on the forward link between the cell site 108 and the wireless unit 110, and a reverse fundamental code channel is established on the reverse link. A fundamental channel (circuit switched link) is maintained throughout the duration of the call and can carry voice, circuit switched data and/or packet data. The selection and distribution unit (SDU) 111 routes the high priority voice traffic from the circuit switched link to the MSC 106.
The wireless units 110 communicate with packet data networks 104 by establishing packet switched connections over the wireless network with the PDN 104. Multiple packet switched connections share wireless network resources to establish a packet switched path between the wireless units 110 and the PDN 104. A packet switched link is established between the wireless units 110 and the base station 108 by a burst management system which coordinates the sharing of available wireless resources, such as wireless channels, among multiple packet switched connections. In current cellular CDMA systems, the fundamental channel and/or one or more supplemental channels can be temporarily assigned to packet switched connections to form the packet switched link. The selection and distribution unit (SDU) 111 routes the lower priority packet data traffic from the packet switched link to a data interworking function (IWF) 112. The IWF 112 provides the interface between the wireless system and the PDN 104, such as the Internet.
For transmission of packet data in the wireless unit 110 to PDN 104 direction over the packet switched link, the wireless unit 110 requests wireless resources, such as supplemental code channels, to support the packet switched connection. For transmission of packet data in the PDN 104 to wireless unit 110 direction, the IWF 112 makes the requests for wireless resources for the packet switched connection. The burst management system collates the burst requests and temporarily assigns the available wireless resources to the packet switched connection. The burst management system uses reported or measured radio environment information to make decisions on the burst allocation strategy, burst size and duration. In doing so, the burst management system takes advantage of the delay-tolerant nature of data to more efficiently use the wireless resources and/or to ensure that sufficient wireless resources remain to handle the higher priority voice traffic. After the temporary allocation of the wireless resources lapses, the packet switched connection must again make a request for wireless resources to be able to transmit packet data.
For the wireless unit 110 to communicate with a device in the PDN 104, a series of rules exists that enable the wireless network and the device on the PDN to understand each other. These rules are called protocols. Because of the large amount of issues which must be resolved for the communications between devices to take place, layered protocols are used which divide the resolution of these issues between layers in the protocol. Each layer operates independently to resolve certain issues to enable the communication between the devices. At the lowest layer is the physical layer which comprises a physical medium(s) between the wireless unit 110 the device in the PDN 104, such as wireless channels, twisted pair, optical cables, and/or coaxial cables. On the physical layer, the information signals simply take the form of bits. Error correction of the communications signals sent over the physical medium is handled by the link layer, such as point-to point protocol (PPP), asynchronous transfer mode (ATM), and frame relay, which group the information sent over the physical layer into frames. To format addresses for the information being transmitted and ensure the information proceeds to the proper destination, a network layer, such as Internet Protocol (IP) is used for routing of data packets. A transport layer, such as transport control protocol (TCP), is used to establish a data session between two devices and to determine if data packets are lost in transit. The transport layer provides also provides recovery from a broken transport or data session. In data communications, the transport layer is performed at the endpoints of the communications, and the information from the endpoint devices is passed down to the lower layers for transmission. Each layer interfaces with the layer immediately below it, and the layer below perform a service to the layers above it. The interfaces between the layers is throughly defined, providing a specific format for the information sent between layers.
In the context of communications between the wireless unit 110 and the packet data network 104, additional issues, such as the transmission of data packets over the wireless channels, must be resolved. A radio link protocol resolves issues related to transmitting data packets over the wireless channels. When a wireless unit 110 requests to make a packet data connection or call, a link layer connection, such as a PPP link, is created in the IWF 112. The link layer connection can be controlled using the PPP Link Control Protocol (LCP), such as defined in Internet Engineering Task Force (IETF) Request for Comment (RFC) 1661. The link layer manages the flow of data leaving the transmitting device and performs error correction for the data packets at the receiving device. As such, the link layer connection acts as the interface for the wireless unit between the wireless communications system and the PDN 104.
Packet data communication on the Internet is dominated by traffic transported using TCP/IP in which data is transmitted using data sessions, referred to as TCP sessions or connections, in a request/response fashion. Before the wireless unit 110 can establish a data session, the link layer connection must be established. Once the link layer connection is established, a requesting device, such as the wireless unit 110, can attempt to establish a TCP session with a responding device, such as a web server on the PDN 104. For example, when a user requests a web page containing embedded images, a number of TCP sessions will typically be required to transmit information between the wireless unit 110 and the web page server using TCP/IP. In typical current usage, all of these TCP sessions must be set up or established before data or information can transmitted. Further background information on TCP/IP can be found in W. R. Stevens, TCP/IP Illustrated, Vol. 1 (Addison-Wesley, 1994).
FIG. 2 shows packets being exchanged over a TCP session initiated by a user. The bold arrows indicate data transfer while thin arrows show synchronization or acknowledgment packets. The TCP session is established with a three-way handshake between the user, such as the wireless unit 110, and the network device, such as a server, as will be described below. To set up the TCP session initiated by a user, the user sends a request to the network device to open a TCP session with the network device. The initial requesting packet does not contain data other than information in the header used in establishing the TCP session, such as a sequence number and a synchronization (SYN) flag which is set to indicate that the TCP session is in the process of being synchronized.
The network device responds by sending a response packet to acknowledge the initial request from the client. In responding to the initial request from the user, the network device sets an acknowledgment (ACK) flag in the header of the response packet, adds one to the sequence number from the user, and puts the modified sequence number into an acknowledgment field of the response packet. The SYN flag is set to indicate that synchronization is not yet complete. When the user receives the response packet from the network device, the user acknowledges the network device's response with an acknowledgment packet. In acknowledging the response packet, the user adds one to the network device's sequence number and puts that sequence number in an acknowledgment field of the acknowledgment packet. The user sets the ACK flag, but the SYN flag is no longer set. After this three-way handshake, the network device and the user continue to acknowledge each other's transmissions, but data can now be transmitted.
The round trip time (RTT) for setting up the TCP session is the amount of time required to send the request packet and receive the response packet. Because TCP session setup is typically associated with every TCP session, frequent TCP setup and tear-down or disconnect can adversely effect the data throughput. For relatively small request-response type packets, the packet switched connection must still request resources and be allocated them, and further data cannot be sent until the data session is set up. When the TCP session is associated with wireless communications, the RTT increases due to the delay associated with sending the setup packets for the data session over the packet switched link between the wireless user 110 and the base station 108. The additional delay arises because the burst management system must receive requests for and temporarily allocate wireless resources to send and receive the setup packets over the packet switched link between the wireless user 110 and the base station 108. The increased RTT can throttle the throughput and data rate. For example, in the wireless environment where the TCP setup time can be 280 ms or more, the setup time may far exceed the data burst time. Such a result contributes to increased delay for most users and degrades the overall data rate and throughput of data communications through the wireless network.